Fuel cells generate electricity from an electrochemical reaction between a hydrogen-containing fuel and an oxidant. One type of fuel cell is a proton-exchange-membrane (PEM) fuel cell, which uses a proton conductive membrane such as NAFION® to separate the fuel and oxidant reactants. Other known fuel cells include solid oxide fuel cells (SOFC), alkaline fuel cells and direct methanol fuel cells (DMFC). Such fuel cells can be stacked together to provide a greater voltage than can be generated by a single fuel cell.
Because fuel cells generate electricity electrochemically rather than by combustion, pollutants found in combustion products can be avoided, and fuel cells are perceived to be an environmentally friendlier alternative to combustion engines. Applications for fuel cells include stationary and portable power generators, and vehicular powerplants.
Especially in vehicular applications, the load on the fuel cell stack can vary dramatically over an operating cycle. Efforts have been made to develop efficient “load-following” fuel cell systems, which can quickly increase or decrease electrical output to match the load changes demanded by the application. However, load-following tends to impose stresses on the fuel cell system, thereby increasing wear and tear on the fuel cell system components and decreasing system operating life.
One approach to reducing the stress on fuel cell systems used in variable load applications is to couple the fuel cell stack in parallel to an energy storage device, such as an electrochemical battery, to produce a “hybrid” power system. In such an arrangement, the battery acts like a buffer for the fuel cell stack, supplying electricity in times of high demand, thereby reducing the peaks in electrical demand on the fuel cell system; when demand is low, the fuel cell stack can recharge the battery. Therefore, the load variations imposed on the fuel cell stack are smoothed and system operating life can be extended.
FIG. 1 illustrates an electrical schematic of a prior art hybrid fuel cell system 91 comprising a fuel cell stack 92, voltage conversion equipment 93, and a battery packet 94, all connected in parallel, and power distribution conductors 95, 96 to allow the parallel connection of a load. The fuel cell stack 92 is capable of generating electricity, provided that fuel and oxidant (collectively, “reactants”) are supplied, as is well known for fuel cell stacks. The voltage conversion equipment 93 is typically a DC/DC voltage converter that has a pulse width modulator, as is well known for direct current voltage regulation. The fuel cell generator, voltage regulation equipment, battery pack and load are coupled in parallel so that the both the fuel cell generator and the energy storage device can provide power to the load, and the fuel cell generator can provide energy to the energy storage device. The battery pack 94 has typically been provided to power peak load demands and to provide power to start the fuel cell generator 92.
There are challenges with implementing a battery hybrid fuel cell system as shown in FIG. 1. One of the most significant challenges is determining the state of charge of the battery. Typically, the battery's state is determined by measuring the current draw on the battery; however this approach does not provide a precise measurement of the battery's charge state, and therefore, it is difficult to precisely determine when and how much the battery needs to be charged by the fuel cell stack. Furthermore, electrochemical batteries do not have a particularly fast discharge rate, and thus sometimes may be not be able to meet the power demands by the load. Another disadvantage of using a battery in such a hybrid configuration is that the battery has a relatively slow recharge rate, and thus may not be able to be recharged quickly to supply power to rapidly variable loads.
There is thus a need to provide an effective fuel cell system that can supply power to highly variable loads in a way that does not unduly stress the fuel cell stack and reduce its operating life.